The present invention relates to a novel material for seats of valves used for controlling the flow of water, oil or gas pertaining to the ordinary life field and/or for regulating the flow of any fluid used in the field of the manufacturing industry such as chemical industry, petrochemical industry, pharmaceutical industry, drink and food industry, etc. and, more particularly, relates to a material for ball seats attached to ball valves.
Generally, the ball seats of this type are made of polytetrafluoroethylene (hereinafter referred to briefly as "PTFE") solely or in combination with glaass fiber, carbon fiber or such metal powder as bronze powder. Further, as disclosed in Japanese Utility Model Publication No. 58-51481, ball seats molded of a copolymer of tetrafluoroethylene and perfluoroalkylvinylether (hereinafter referred to simply as "PFA") are also known to the art.
As is well known, PTFE exhibits considerably low frictional resistance, has a particular material characteristic of its static friction being not greater than its dynamical friction, and is excellent in resistance to wear, chemicals and temperature. This is why PTFE has found widespread acceptance as a material for seats of ball valves. For this reason, PTFE is specified as a material for ball seats in The Japan Petroleum Institute's Standard, "Flanged Ball Valves for Petroleum Industry" (JPI-7S-48-74) and also in The British Standard, "Steel ball valves for petroleum, petrochemical and allied industries" (BS 5351:1976).
However, the ball seats made of PTFE are inferior in resistance to creep and, when being used under high pressure, gives rise to creep, thereby undergoing permanent deformation and resulting in malfunction of the associated ball valves. In order to avoid such malfunction, it is required to place restrictions on the maximum pressure to be used. Particularly, since the larger a valve bore, the larger a force exerted on a ball seat, the maximum pressure to be used must be restricted to a much lower level. The maximum pressure usable for each of the standardized ball seats is also specified in The Japan Petroleum Institute's Standard and The British Standard. Under the specified maximum pressure, however, problems resulting from creep still remain.
Why resistance to creep is necessary for ball valves will now be described.
When pressure is applied onto the upstream side of a ball valve kept closed, for example, it acts on a ball of the ball valve and consequently the ball compresses a ball seat on the downstream side of the ball valve. This compressing force causes deformation of the seat and consequently creep of the seat. After the downstream seat has undergone such creep as described above, when the application of pressure onto the upstream side is released, the ball supported between the downstream seat and a ball seat on the upstream side of the ball valve with the aid of their elastic deformation will descend by an amount corresponding to the creep quantity. In this state, when pressure is again applied onto the upstream side, a fluid leaks toward the downstream side through a gap formed by the descent of the ball. At this time, the ball valve has lost its sealing function. At the time of first application of pressure onto the upstream side, there is a fair possibility of the upstream seat being shifted from its fixed position toward a valve chamber between the ball and a valve body, with the result that the upstream seat undergoes creep. In this case, the upstream seat cannot support the ball thereon and, when the application of pressure onto the upstream side is released, the ball is allowed to descend. In order to avoid the aforementioned adverse phenomena, it is necessary for the seats to be protected from creep.
In view of the drawbacks suffered by PTFE, so-called reinforced PTFE has recently found widespread acceptance. The reinforced PTFE is formed by admixing glass fiber or carbon fiber with PTFE as described above and is superior in resistance to creep to PTFE per se. However, the superiority in resistance to creep is not so completely satisfactory. In addition, the cost of the PTFE reinforced with glass fiber and the cost of the PTFE reinforced with carbon fiber are about 1.3 times and about 2 times respectively that of PTFE per se.
Further, when the flow of such monomers as styrene is controlled with a ball valve having ball seats of PTFE or reinforced PTFE, the styrene permeates the seats and is polymerized therein to cause embrittlement of the seats, thereby giving rise to breakage of the seats and malfunction of the ball valve.
On the other hand, the aforementioned copolymer "PFA" is very excellent as a material for ball seats because it is difficult to permit permeation of such monomers as styrene, exhibits high resistance to creep, is capable of being used under high pressure and, even under the aforementioned maximum pressure usable, does not suffer from the problems resulting from creep and encountered by PTFE or reinforced PTFE. However, the cost of PFA is 5 to 10 times that of PTFE, resulting in very expensive products.
Under these circumstances, in this technical field there is now an increased demand for inexpensive ball seats excellent in resistance to creep and also in resistance to wear.